The present invention relates to the cryogenic separation of a feed gas mixture to provide at least one product gas mixture having a different composition from the feed mixture and has particular, but not exclusive application, to providing a product gas mixture containing a desired molar ratio of hydrogen and carbon monoxide from a gaseous feed, especially syngas, containing them in a different molar ratio.
Syngas is a gaseous mixture consisting primarily of hydrogen and carbon monoxide which, depending upon the level of purity, can contain small amounts of argon, nitrogen, methane and other trace hydrocarbon impurities. Usually, it is obtained by catalytic conversion or partial oxidation of coal, coke, natural gas, or other hydrocarbon feeds. The primary uses of syngas are in the synthesis of methanol (requiring hydrogen:carbon monoxide molar ratio of 2:1) and in reactions to produce oxo-alcohols (requiring hydrogen:carbon monoxide molar ratio of at least 1:1). For many of these applications, it is necessary to control the relative proportions of hydrogen and carbon monoxide. This is achieved by, for example cryogenically separating crude syngas into hydrogen-rich and carbon monoxide-rich product streams and then combining them in the appropriate molar ratio to produce the required syngas composition. The level of impurities, especially methane and other hydrocarbons, in the crude syngas usually also is reduced during the cryogenic separation.
It is known to integrate cryogenic and membrane separation techniques to separate feed gas mixtures but in the prior art the membrane separation is used to enrich the feed stream to the cryogenic separation . In the case of syngas separation, the prior art integration exclusively uses hydrogen permeation membranes to provide carbon monoxide-enriched feed or recycle streams for feeding to the cold box of the cryogenic separation.
U.S. Pat. No. 4,595,405 (R. Agrawal & S. Auvil; 1986) describes a separation process in which a gas stream from a cryogenic separation is subjected to a membrane separation and at least some of the discharge gas from the membrane separation is recycled to the cryogenic separation. The process is exemplified by the separation of air (or other feed gas mixture of nitrogen and oxygen) to produce gaseous and/or liquid nitrogen in which an oxygen-enriched stream from the cryogenic separation is subjected to the membrane separation to provide an oxygen-rich permeate stream and a nitrogen-enriched recycle stream.
U.S. Pat. No. 4,654,063 (S. Auvil & R. Agrawal; 1987) describes integration of a membrane separation with a cryogenic or other non-membrane separation to recover hydrogen from a feed gas mixture. The membrane separation is used to remove hydrogen from the feed to the non-membrane separation and/or from a hydrogen enriched stream produced in the non-membrane separation prior to recycle of the resultant hydrogen-lean stream to the non-membrane separation.
U.S. Pat. No. 4,654,047 (J. Hopkins et al.; 1987) describes a process for obtaining hydrogen from a feed gas in which the feed gas is subjected to membrane separation upstream of cryogenic separation to provide a hydrogen-lean feed to the cryogenic separation. A hydrogen-rich stream from the cryogenic separation is recycled to the membrane to recover additional hydrogen as product.
JP-A-63-247582 (Y. Tomisaka; 1988) describes a process to separate carbon monoxide from feed containing predominantly carbon monoxide and hydrogen in which the feed is subjected to a membrane separation immediately upstream of a cryogenic separation to raise the concentration of carbon monoxide in the gas fed to the cryogenic separation.
FR-A-2 636 543 (P. Gauthier & C. Monereau; 1990) describes an integrated system for producing ammonia synthesis gas (hydrogen & nitrogen) in which a membrane separation removes excess hydrogen upstream of a cryogenic purification system. Only the gas feed to the cryogenic system is processed by the membrane.
EP-A-0 359 629 (P. Gauthier; 1990; see also corresponding AU-A-41236/89) describes the use of a permeator to remove excess hydrogen from a syngas to adjust the H.sub.2 :CO molar ratio prior to feeding to a cryogenic separation for the production of hydrogen and carbon monoxide.
DE-A-43 25 513 (R. Fabian; 1994) describes a process for recovery of a high purity carbon monoxide product stream and a hydrogen product stream using a membrane integrated with a cryogenic partial condensation cycle. An intermediate syngas stream is passed through a membrane to remove hydrogen before the stream is recycled to the cryogenic system to recover and purify the carbon monoxide product. The claimed benefit relative to a traditional condensation cycle is the elimination of the cold heat exchanger and hydrogen expansion refrigeration system.
GB-A-2 282 082 (J. Gilron & A. Soffer; 1995) describes integration of a membrane directly into a cryogenic process with the membrane itself run at cryogenic temperatures. The stated advantage is the elimination of the thermodynamically inefficient process of warming a gas stream from the cryogenic unit, passing it through the membrane at ambient conditions, and recooling the membrane discharge gas for further cryogenic processing. Specific focus is toward improving the membrane-cryogenic integrations described in U.S. Pat. No. 4,654,063 and U.S. Pat. No. 4,595,405.
The prior art integration of cryogenic and membrane separation techniques in syngas separation (in which hydrogen permeation membranes provide carbon monoxide-enriched feed or recycle streams to the cryogenic separation) improves overall efficiency and/or capacity of the cryogenic separation but does not address the growing complexity and control required of new syngas processing facilities. Recently there has been an increasing demand for simultaneous production of carbon monoxide, hydrogen, and one or more hydrogen/carbon monoxide products under varying feed composition and product slate scenarios. The present invention is targeted toward such simultaneous production requirements. In particular, it is an objective of this invention to improve the control and versatility of gas mixture separation processes using cryogenic separation to produce a gas mixture product of different composition to the feed gas mixture. More particularly, it is an objective of the invention to provide a separation process which is capable of improving the control and versatility of a cryogenic condensation separation of syngas to produce carbon monoxide, hydrogen and one or more hydrogen/carbon monoxide gas mixture products through the integration of a product (and optional feed conditioning) membrane.